why is ovs agent writing flows to both external and internal bridges?

In ovs_neutron_agent.py, under function provision_local_vlan, there is a section :
# outbound
                br = self.phys_brs[physical_network]
                br.add_flow(priority=4,
                            in_port=self.phys_ofports[physical_network],
                            dl_vlan=lvid,
                            actions="mod_vlan_vid:%s,normal" % segmentation_id)
# inbound
               self.int_br.add_flow(priority=3,
               in_port=self.
                                     int_ofports[physical_network],
                                     dl_vlan=segmentation_id,
                                     actions="mod_vlan_vid:%s,normal" % lvid)

Let's say I created a tenant network with vlan 3. I understand the inbound bridge should allow the traffic. But when outbound bridge is also provisioned with this vlan tag, you are creating a loop! What is the point for physical network to get private network traffic?